This program is a study of the immune parameters, both humoral and cellular, associated with the expression of and/or protection from Marek's disease tumors. It has been established so far that vaccination with a mild strain of Marek's disease virus enhances protection against tumor development and that this protection may be associated with suppression of the replication of an oncogenic Marek's disease virus. Although certain differences in humoral antibody parameters and lymphocyte functions have been observed in a limited number of tests, the work and data in this area is yet too limited for definite conclusions.